In order to obtain a higher capacity lead-acid battery and to facilitate the production process of the lead-acid battery, an expanded grid is mostly used these days because an expanded grid can be made thinner than a conventional grid obtained by casting and it can be serially produced.
In a lead-acid battery, the volume change of the active material during charging/discharging is relatively large. Accordingly, the repetition of charging and discharging weakens the binding force within the active material, making it easier for the active material to separate from the electrode.
In the case where the current collector is a casting grid, it is easy to form a rigid frame around the electrode. The volume change of the active material can be suppressed to a certain degree by filling the inside of the frame with an active material; accordingly, the separation of the active material can be prevented.
In the case where the current collector is an expanded grid, on the other hand, unlike the case of a casting grid, it is difficult to form a frame around the electrode because of its production method. Therefore, the active material filling the grid at the right and left edge portions of the electrode is not surrounded by a frame. As shown in FIGS. 1 and 2 which will be used in examples described hereinafter, in a negative electrode 5 comprising an expanded grid 1 and an active material layer 2, the active material layer 2 is exposed outside at the right and left portions of the electrode. Thus, the active material at these portions of the electrode is likely to separate from the electrode due to the volume change of the active material during charging/discharging. The separated active material deposits on the lower portion of the electrode, which is a cause of short circuit between the positive and negative electrodes. This internal short circuit degrades the battery characteristics.
In a valve regulated lead-acid battery, there are cases where the active material grows abnormally at the right and left edge portions of the negative electrode during the repetition of charging and discharging. The problem arises that the active material grows and reaches the positive electrode, causing an internal short circuit.
A valve regulated lead-acid battery has a system of reducing an oxygen gas generated at the positive electrode into water at the negative electrode during charging, which prevents the electrolyte from going away outside the system. This system is a cause of the abnormal growth of the active material.
First, oxygen gas generated at the positive electrode reaches the surface of the negative electrode and is reduced to water by metallic lead of the negative electrode. Meanwhile, the metallic lead of the negative electrode which has reduced the oxygen gas is oxidized to a lead oxide. Subsequently, the lead oxide is dissolved in the electrolyte and is reacted with sulfuric acid to give lead sulfate. The lead sulfate is reduced to metallic lead by receiving electrons at the negative electrode.
When the oxygen gas is reduced, metallic lead is required to have a solid-gaseous interface because metallic lead causes solid-gaseous phase reaction with the oxygen gas. Further, the produced lead oxide is reduced to lead sulfate and then to metallic lead; thereby, it is possible to continuously cause the reduction reaction with the oxygen gas. Accordingly, it is of importance that the metallic lead also has a solid-liquid interface.
In view of the above, it is considered that the area where the oxygen gas is efficiently reduced into water is the right and left edge portions of the negative electrode having larger three-phase interfaces (solid, liquid, gas). Accordingly, an apparent charge and discharge reaction occurs more at the right and left edge portions of the negative electrode than other portions; inevitably, the volume change of the active material is significant at the edge portions, making it easier for the active material to separate.
Moreover, since the reduction reaction of the oxygen gas is a reaction that accompanies dissolution and deposition, the shape of the active material changes significantly. Therefore, the abnormal growth of metallic lead is likely to occur at the right and left edge portions of the negative electrode.
In order to solve the above problems, for example, Japanese Patent No. 2742804 proposes a method in which a positive electrode plate is encased in a bag-shaped or U-shaped mat separator composed mainly of glass fiber and a negative electrode plate is encased in a bag-shaped separator made of polymer resin.
Further, Japanese Patent No. 3146438 proposes a method to prevent the separation of an active material and an internal short circuit by filling the space between positive and negative electrode plates and the periphery of the electrode plates with powdered silica.
According to the method described in Japanese Patent No. 2742804, a separated active material can be maintained at a certain position, and an internal short circuit resulting from the separation of an active material can be sufficiently prevented. However, in order to ensure the mechanical strength of a separator made of polymer resin in the process to form the separator into a bag shape, the separator is required to have a certain thickness. If such separator made of polymer resin is interposed between the positive electrode and the negative electrode, the space between the positive electrode and the negative electrode will be widened. This increases the resistance of electrolyte, which degrades the output characteristics of the battery. What is worse is that this method cannot prevent the separation of the active material itself. Thus, as charging and discharging are repeated, the amount of the active material not involved in charging and discharging increases, which decreases the battery capacity.
Moreover, according to a method described in Japanese Patent No. 3146438, it is difficult to efficiently fill a battery container with powdered silica, and a significant effect cannot be expected. What is worse is that the permeation of an oxygen gas or the like generated during charging is inhibited. Therfore, it is possible that the reduction reaction of an oxygen gas is inhibited, which causes the electrolyte depletion.
Furthermore, since the above two methods use a large amount of polymer resin or powdered silica which is not involved in charging/discharging, the space equal to the volume of polymer resin or powdered silica is wasteful and the amount of the active material useful for charging/discharging is reduced.
In order to solve the above problem, it is an object of the present invention to suppress an internal short circuit resulting from the separation or abnormal growth of an active material, thereby providing a lead-acid battery with longer life.